It is well known how to prepare olefin polymer foams by heat plastifying a normally solid olefin polymer resin, admixing such heat plastified resin with a volatile blowing agent under heat and pressure to form a flowable gel and, thereafter, extruding the gel into a zone of lower pressure and temperature to activate the blowing agent and expand and cool the gel to form the desired solid olefin foam product.
A problem frequently encountered is shrinkage or expansion of such foam products in fresh or partially cured form (i.e. prior to aging to permit substantially complete release and/or outgassing of the blowing agent from within the cellular structure of the foam). During the aging or curing period, the blowing agent employed gradually diffuses out of the cells in the foam product and air gradually diffuses into the cells in place thereof. There was a time when it was believed that only a halogenated hydrocarbon blowing agent, namely dichlorotetrafluoroethane, was capable of providing sufficient dimensional stability during the curing period to permit the commercially viable manufacture of low density foams of ethylenic polymer resins. That is, only dichlorotetrafluoroethane was believed to diffuse out of the foam cells slowly enough to prevent cell wall collapse while air was slowly diffusing into the cells when the foam was processed without a permeability modifier or stability control agent.
Heretofore permeability modifiers or stability control agents were developed for incorporation into the polyolefin in an attempt to slow the diffusion of blowing agents out of polyolefin foam cells and thereby minimize shrinkage of the foam cells. For purposes of this application, the terms "permeability modifier" and "stability control agent" will be used interchangeably.
As discussed in the following patents, there are several different types of stability control agents used for minimizing cell shrinkage or cell collapse in foam.
U.S. Pat. No. 3,644,230 discusses a method for preventing post extrusion cell collapse in polyolefin foams by the incorporation of a small amount of partial esters of a long chain fatty acids with polyols.
U.S. Pat. No. 3,755,208 discusses a method for preventing post extrusion cell collapse in vinyl copolymer foams by the incorporation of a small amount of partial esters of long chain fatty acids with polyols.
U.S. Pat. No. 4,214,054, discusses the production of polyolefin foams utilizing volatile hydrocarbon blowing agents. Permeability modifiers such as saturated higher fatty acid amides, saturated higher aliphatic amines, and complete esters of saturated higher fatty acids are incorporated into the polyolefin composition prior to foam expansion.
U.S. Pat. No. 4,217,319 discusses the use of a volatile organic as a blowing agent to expand a polyolefin with an ester of a long chain fatty acid and a polyhydric alcohol as a permeation modifier or stability control agent to prevent shrinkage of the expanded polyolefin.
U.S. Pat. No. 4,331,779, discusses ethylenic polymer foams having a copolymer of ethylene with an unsaturated carboxylic acid as a stability control agent.
U.S. Pat. No. 4,347,329, discusses the use of a fatty acid amide such as stearamide for use in polyolefin foams as a stability control agent. U.S. Pat. Nos. 4,368,276 & 4,395,510 further discusses the use of fatty acid amide stability modifier agents to produce polyolefin foams having improved elevated temperature dimensional stability.
U.S. Pat. Nos. 4,640,933; 4,633,361; 4,694,027 discuss the use of a fatty acid amide such as stearamide as a stability control agent for use in polyolefin foams blown with blowing agents selected from the group consisting of (i) isobutane, (ii) a mixture of from 5%-95% isobutane on a molar basis with from 95%-5% of a physical selected from the group consisting of chlorofluorocarbons and fluorocarbons and (iii) a mixture of at least 70% isobutane with other hydrocarbons, chlorocarbons and chlorofluorocarbons.
U.S. Pat. No. 5,424,016 discusses the use of permeability modifiers, such as fatty acid amides and esters, in a quantity sufficient to prevent substantial shrinkage of the foam structure.
The use of such permeability modifiers permit the use of a wider variety of volatile blowing agents. In most cases, the more inexpensive blowing agents, such as isobutane, can only produce viable foams when employed in conjunction with stability control agents.
Although the stability control agents discussed in these patents may be helpful in minimizing the shrinkage of the polyolefin foams blown with volatile organics, neither the foam systems nor the stability control agents heretofore address the problem of post-cooling expansion. For the purposes of this application, the term "post-cooling expansion" will refer to expansion of the foam, in atmospheric conditions, after the foam has cooled. While not wanting to be held to any particular theory, it has been said that post-cooling expansion results from the residual blowing agent releasing from the polymer structure defining the cells into the open area of the cell at a rate faster than it is able to diffuse from the cell.
The post-cooling expansion contributes to a multitude of problems. The post foaming processing of the foam is effected by the change in dimensions and stress resulting from this post-cooling expansion. In particular, whether the foam is extruded into planks, sheets or some other forms, a dimensional change in the foam results in inconsistency and unpredictability in articles formed therefrom. This effect is particularly acute in laminated forms where the post-cooling dimensional change in the foam may provide a cumulative change.
When the foam is wound as sheet on a roll, the layers of foam on the outer portion of the roll tend to expand in turn compressing the inner layers so that a significant amount of the foam is less than its extruded thickness. As an example, a sheet extruded at a 0.250" thickness and wound as a roll may grow to a thickness of about 0.300" in the outer layers, while the inner layers will be compressed to a thickness of about 0.200". Depending on the particulars of the foam type, there may be as much as 50 to 60% of the roll compressed to a thickness below the cooled extruded foam thickness. This variation in thickness and stress in the rolled foam adversely effects the processing of sheets cut from the roll. Specifically, after the foam is freed from the roll and cut, it may tend to grow, shrink and/or relax resulting in changes in thickness, width and/or length of the cut sheet.
Accordingly, the need still exists in the art for relatively low density polyolefin foams that exhibit a high degree of post-cooling dimensional stability with minimal shrinkage and expansion. This, and other objects and advantages of the invention will become apparent from the following detailed description and the appended claims.